This invention relates generally to metal detection apparatus, and more particularly the invention relates to a system which detects signals emitted from a metal object such as an underground pipe.
In the construction and utility maintenance fields, the ability to trace underground metal and underground piping has been needed, for both safety and maintenance purposes. Commonly available equipment to do this job uses very low frequency systems for detection and tracing.
The ability to detect and trace underground piping is greatly affected by the environment that exists at the time of detection. If the ground is very wet or is made of different combinations of earth, erroneous results may occur. If the underground pipe or metal contains an insulating gasket, this may limit the distance at which the pipe can be traced due to the gasket acting like a very low value capacitor. This capacitor is a high impedance to the very low frequency signals, thus preventing them from jumping across the gasket. In addition, the ability to couple the low frequency energy to the underground metal or pipe becomes very difficult. The extreme low frequency makes necessary a very high voltage for the pipe to emit enough signal to be detected.
Display systems for construction utility maintenance tracing equipment are generally single point displays. This means that they can accurately indicate the position of a single pipe or piece of metal. In co-pending application Ser. No. 09/734,963, supra, the technique of time of arrival or phase differential detection is used. In this system, the phase difference between two arrival signals is measured to give positional information to a single point detector. A single point detector is defined as a device that indicates a peak or null when over the single target. Multiple targets can give erroneous readings.
The present invention is directed to a metal locating system which will accurately show the position of multiple targets simultaneously. The system can be used over a wide frequency range, from the very low frequencies to ultra high frequencies.
The present invention includes a transmitter section for applying an RF signal to a metal or underground pipe which then acts as a transmitter, and a receiver section including a sequentially activated antenna array with each antenna of the array capable of detecting the transmitted signal. The antenna array effectively frequency modulates the detected signal depending on orientation of the object from the array.
More particularly, the transmitter section comprises a radio frequency oscillator and a radio frequency power amplifier. The output of the radio frequency power amplifier is connected to an impedance matching network. This network is used to turn the pipe or metal being driven into a loss line transmission radiator.
The receiver section of the invention comprises an array of signal sensors or antenna elements. This array can be any number of elements in size. For explanation purposes, 4 and 8 element arrays are used. The signal sensor elements are selected by an electronic sensor selection switch. The output of this switch feeds an FM receiver. The output of the FM receiver feeds a lowpass filter and a high Q bandpass filter. These can be switched capacitor filters and their clock frequencies are synchronized with the signal sensor selection clock. The output of the bandpass filter feeds a negative slope zero crossing detector. A zero crossing corresponds to an antenna pair parallel to and coplanar with the radiating pipe. A negative slope zero crossing detector outputs a pulse on each negative going zero crossing at its input. The output of the negative slope zero crossing detector feeds an adjustable delay line. The adjustable delay line corrects timing errors in the system. The output of the delay line feeds the vector display unit. A master clock generator synchronizes all circuits and provides a reference clock for the vector display unit.
The signal sensor elements must all be the same type. A four element signal sensor array consists of identical sensor elements placed at the end of a symmetrical cross. The eight element signal sensor array has 8 identical sensor elements placed in a circular fashion. The sensor elements for either array are selected in by an electronic switch. The action of the switch is the equivalent to mechanically rotating a single sensor element. The output from the signal sensor selector switch feeds an FM receiver. The output of the FM receiver is a series of audio pulses, each one corresponding to a sensor element of the signal sensor array during electronic selection. The output signal is fed to a filter system which converts the pulses to an analog signal and cleans the signal before being then applied to the high Q bandpass filter. The high Q bandpass filter has a clock signal that is synchronized to the switching control signal of the signal sensor array electronic switch. This synchronization provides consistent data to the negative slope zero crossing detector. The negative slope zero crossing detector accurately detects the frequency shift of the signal caused by the Doppler shift in the signal sensor array. This is done by measuring the shift in the negative slope zero crossing detector against the reference clock signal of the clock system. To synchronize the timing and make up for delays in the system, an adjustable delay line is inserted between the output of the negative slope zero crossing detector and the vector display unit. This delay line allows the positional calibration of the vector display unit. Each light of the vector display corresponds to a positional vector and to a detected zero crossing.
When the signal sensor array is to the left of the target pipe, the vector display indicates how far to the left the signal sensor is. As the signal sensor array is moved over the target, the position of the target on the display will change with the position of the array over the target and the position of the detected zero crossing. If the target pipe is a branch, both sections of the branch will appear on the vector display. As previously stated, the commutation of the electronic sensor switch is equivalent to physically rotating a single signal sensor element at high-speed. The Doppler effect of this rotation is the detected frequency at the output of the FM receiver. The zero crossing of the detected frequency at the output of the negative slope zero crossing detector, accurately defines the position of the target. This information is displayed on the vector unit.